Weather patterns across the globe are partly affected by
connections between the 11-year solar cycle of activity, Earth's stratosphere
and the tropical Pacific Ocean, a new study finds.
The study could help scientists get an edge on eventually
predicting the intensity of certain climate phenomena, such as
the Indian monsoon and tropical Pacific rainfall, years in advance.
The sun is the ultimate source of all the energy on Earth;
its rays heat the planet and drive the churning motions of its atmosphere.
The amount of energy the sun puts out varies over an
11-year cycle (this cycle also governs the appearance
of sunspots on the sun's surface as well as radiation storms that can knock
out satellites), but that cycle changes the total amount of energy reaching
Earth by only about 0.1 percent. A conundrum for meteorologists was explaining whether
and how such a small variation could drive major changes in weather patterns on
Earth.
Earth-space connection
An international team of scientists led by the National Center for Atmospheric Research (NCAR) used more than a century of weather
observations and three powerful computer models to tackle this question.
The answer, the new study finds, has to do with the Sun's
impact on two seemingly unrelated regions: water in the tropical Pacific Ocean and air in the stratosphere, the layer of the atmosphere that runs from
around 6 miles (10 km) above Earth's surface to about 31 miles (50 km).
The study found that chemicals in the stratosphere and sea
surface temperatures in the Pacific Ocean respond during solar maximum in a way
that amplifies the sun's influence on some aspects of air movement. This can
intensify winds
and rainfall, change sea surface temperatures and cloud cover over certain
tropical and subtropical regions, and ultimately influence global weather.
"The sun, the stratosphere, and the oceans are
connected in ways that can influence events such as winter rainfall in North America," said lead author of the study, Gerald Meehl of NCAR. "Understanding
the role of the solar cycle can provide added insight as scientists work toward
predicting regional weather patterns for the next couple of decades."
The findings are detailed in the Aug. 28 issue of the
journal Science.
How it happens
The changes occur like this: The slight increase in solar
energy during the peak production of sunspots is absorbed by stratospheric
ozone, warming the air in the stratosphere over the tropics, where sunlight
is most intense. The additional energy also stimulates the production of
additional ozone there that absorbs even more solar energy.
Since the stratosphere warms unevenly, with the most
pronounced warming occurring nearer the equator, stratospheric winds are altered
and, through a chain of interconnected processes, end up strengthening tropical
precipitation.
At the same time, the increased sunlight at solar maximum
— a peak of sunspot and solar storm activity we're currently headed toward
— causes a slight warming of ocean surface waters across the subtropical
Pacific, where sun-blocking clouds are normally scarce. That small amount of
extra heat leads to more evaporation, putting additional water vapor into the
atmosphere. The moisture is carried by trade winds to the normally rainy areas
of the western tropical Pacific, fueling heavier rains and reinforcing the
effects of the stratospheric mechanism.
These two processes reinforce each other and intensify the
effect.
These stratospheric and ocean responses during solar maximum
keep the equatorial eastern Pacific even cooler and drier than usual, producing
conditions similar to a La Nina event. However, the cooling of about 1-2
degrees Fahrenheit is focused farther east than in a typical La Nina (the
opposite sister
effect of the warm-water El Nino), is only about half as strong, and is
associated with different wind patterns in the stratosphere.
The solar cycle does not have as great an effect on Earth's
climate as the El Nino cycle.
But the Indian monsoon, Pacific sea surface temperatures and
precipitation, and other regional climate patterns are largely driven by rising
and sinking air in Earth's tropics and subtropics. The new study could help
scientists use solar-cycle predictions to estimate how that circulation, and
the regional climate patterns related to it, might vary over the next decade or
two.
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